Add documentation for how to profile app startup time

[kenzieschmoll]

A user filed an issue requesting a way to profile app startup: https://github.com/flutter/devtools/issues/6975. I responded with some information about how to use DevTools to profile the Dart side, but there is more that needs to be done here in order to get information about the native code that runs before / while the Dart VM starts up. I looked on flutter.dev to see if we have an official guide on this topic and I could not locate one.

Sounds like @jiahaog might be the expert on this topic if we want to put together formal guidance.